C++ || Snippet – How To Read & Write Data From A User Specified Text File

April 29, 2012 admin No comments

This page will consist of a demonstration of a simple quadratic formula program, which highlights the use of the input/output mechanisms of manipulating a text file. This program is very similar to an earlier snippet which was presented on this site, but in this example, the user has the option of choosing which file they want to manipulate. This program also demonstrates how to read in data from a file (numbers), manipulate that data, and output new data into a different text file.

REQUIRED KNOWLEDGE FOR THIS SNIPPET

Fstream Ifstream Ofstream Working With Files C_str() - Convert A String To Char Array Equivalent Getline - String Version

Note: The data file that is used in this example can be downloaded here.

Also, in order to read in the data .txt file, you need to save the .txt file in the same directory (or folder) as your .cpp file is saved in. If you are using Visual C++, this directory will be located in

Documents > Visual Studio 2010 > Projects > [Your project name] > [Your project name]





		
		
			User Specified Input/Output Manipulation
			
C++
			
			#include <iostream>
#include <fstream>
#include <string>
#include <cmath>
#include <cstdlib>
using namespace std;

int main() 
{
    // declare variables
    // char fileName[80];
    string fileName;    
    ifstream infile;
    ofstream outfile;
    double a=0,b=0,c=0;
    double root1=0, root2=0;    
    
    // get the name of the file from the user
    cout << "Please enter the name of the file: ";    
    getline(cin, fileName);
    
    // ^ you could also use a character array instead
    // of a string. Your getline declaration would look
    // like this:
    // ---------------------------------------------
    // cin.getline(fileName,80);


    // this opens the input file
    // NOTE: you need to convert the string to a 
    // char array using the function "c_str()"
    infile.open(fileName.c_str());
    
    // ^  if you used a char array as the file name instead 
    // of a string, the declaration to open the file 
    // would look like this:
    // ---------------------------------------------
    // infile.open(fileName);
    
    // check to see if the file even exists, & if not then EXIT
    if(infile.fail())
    {
        cout<<"nError, the input file could not be found!n";
        exit(1);
    }
    
    // this opens the output file
    // if the file doesnt already exist, it will be created
    outfile.open("OUTPUT_Quadratic_programmingnotes_freeweq_com.txt");  
    
    // this loop reads in data until there is no more 
    // data contained in the file
    while(infile.peek() != EOF)
    {
        // this assigns the incoming data to the
        // variables 'a', 'b' and 'c'
        // NOTE: it is just like a cin >> statement
        infile>> a >> b>> c;

        // NOTE: if you want to read data into an array
        // your declaration would be like this
        // ------------------------------------ 
        // infile>> a[counter] >> b[counter] >> c[counter];
        // ++counter;
    }    
    
    // this does the quadratic formula calculations
    root1 = ((-b) + sqrt(pow(b,2) - (4*a*c)))/(2*a);
    root2 = ((-b) - sqrt(pow(b,2) - (4*a*c)))/(2*a);

    // this displays the numbers to screen via cout
    cout <<"For the numbersna = "<<a<<"nb = "<<b<<"nc = "<<c<<endl;
    cout <<"nroot 1 = "<<root1<<"nroot 2 = "<<root2<<endl;
     
    // this saves the data to the output file
    // NOTE: its almost exactly the same as the cout statement
    outfile <<"For the numbersna = "<<a<<"nb = "<<b<<"nc = "<<c<<endl;
    outfile <<"nroot 1 = "<<root1<<"nroot 2 = "<<root2<<endl;

    // close the input/output files once you are done using them
    infile.close();
    outfile.close();

    // stops the program from automatically closing
    cout<<"nPress ENTER to continue...";
    cin.get();

    return 0;
}// http://programmingnotes.org/
			
				
					
				
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						#include <iostream>
#include <fstream>
#include <string>
#include <cmath>
#include <cstdlib>
using namespace std;
 
int main() 
{
    // declare variables
    // char fileName[80];
    string fileName;    
    ifstream infile;
    ofstream outfile;
    double a=0,b=0,c=0;
    double root1=0, root2=0;    
    
    // get the name of the file from the user
    cout << "Please enter the name of the file: ";    
    getline(cin, fileName);
    
    // ^ you could also use a character array instead
    // of a string. Your getline declaration would look
    // like this:
    // ---------------------------------------------
    // cin.getline(fileName,80);
 
 
    // this opens the input file
    // NOTE: you need to convert the string to a 
    // char array using the function "c_str()"
    infile.open(fileName.c_str());
    
    // ^  if you used a char array as the file name instead 
    // of a string, the declaration to open the file 
    // would look like this:
    // ---------------------------------------------
    // infile.open(fileName);
    
    // check to see if the file even exists, & if not then EXIT
    if(infile.fail())
    {
        cout<<"nError, the input file could not be found!n";
        exit(1);
    }
    
    // this opens the output file
    // if the file doesnt already exist, it will be created
    outfile.open("OUTPUT_Quadratic_programmingnotes_freeweq_com.txt");  
    
    // this loop reads in data until there is no more 
    // data contained in the file
    while(infile.peek() != EOF)
    {
        // this assigns the incoming data to the
        // variables 'a', 'b' and 'c'
        // NOTE: it is just like a cin >> statement
        infile>> a >> b>> c;
 
        // NOTE: if you want to read data into an array
        // your declaration would be like this
        // ------------------------------------ 
        // infile>> a[counter] >> b[counter] >> c[counter];
        // ++counter;
    }    
    
    // this does the quadratic formula calculations
    root1 = ((-b) + sqrt(pow(b,2) - (4*a*c)))/(2*a);
    root2 = ((-b) - sqrt(pow(b,2) - (4*a*c)))/(2*a);
 
    // this displays the numbers to screen via cout
    cout <<"For the numbersna = "<<a<<"nb = "<<b<<"nc = "<<c<<endl;
    cout <<"nroot 1 = "<<root1<<"nroot 2 = "<<root2<<endl;
     
    // this saves the data to the output file
    // NOTE: its almost exactly the same as the cout statement
    outfile <<"For the numbersna = "<<a<<"nb = "<<b<<"nc = "<<c<<endl;
    outfile <<"nroot 1 = "<<root1<<"nroot 2 = "<<root2<<endl;
 
    // close the input/output files once you are done using them
    infile.close();
    outfile.close();
 
    // stops the program from automatically closing
    cout<<"nPress ENTER to continue...";
    cin.get();
 
    return 0;
}// http://programmingnotes.org/

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Once compiled, you should get this as your output

Please enter the name of the file: INPUT_Quadratic_programmingnotes_freeweq_com.txt
For the numbers a = 2 b = 4 c = -16 root 1 = 2 root 2 = -4
Press ENTER to continue...

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Posted in: Basic, C++, C++, C++, Getting Started, Snippets Tags: array, c++, char, string, text file, While Loop

C++ || Snippet – How To Read & Write Data From A User Specified Text File

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